THE SOLAR SYSTEM
Is the group of celestial bodies, including the Earth, orbiting around and gravitationally bound by the star known as the Sun, one of at least a hundred billion stars in our galaxy. The Sun's retinue includes nine planets, about 50 satellites, more than 1,000 observed comets, and thousands of lesser bodies known as minor planets (asteroids) and meteoroids (see meteor and meteorite). All of these bodies are immersed in a tenuous sea of fragile and rocky interplanetary dust particles, perhaps ejected from comets at the time of their passage through the inner solar system or resulting from minor planet collisions. The Sun is the only star known to be accompanied by such an extensive planetary system. A few nearby stars are now known to be encircled by swarms of particles of undetermined size, however (see planets and planetary systems), and other nearby stars are accompanied by planets in the size range of Jupiter or larger (see brown dwarf). Thus the possibility of a universe filled with many solar systems remains strong, though as yet unproved.